Arterial oxygen content was once assessed relying only on physical signs and symptoms such as cyanosis, tachypnea, bradycardia, tachycardia, dyspnea, and shortness of breath. Today, devices exist which allow accurate and rapid quantitative measurement of arterial oxygen content. Partial pressure of oxygen (PO2) in blood, percent hematocrit (Hct), percent arterial hemoglobin saturation (SaO2), gram-percent total hemoglobin (THb), and arterial oxygen content (CaO2) are all readily available to the physician in modern hospitals.
Unfortunately, however, measurement of these variables has until recently always required an invasive arterial puncture or phlebotomy. Once the whole blood sample is obtained, analysis is accomplished using spectrophotometric and chemical means.
During the early 1970's the first pulse oximeter was introduced. This device permitted approximation of SaO2, termed SpO2, by nonivasive means. The design was subsequently improved upon and the current generation of pulse oximeters is now commonplace in the intensive care unit (ICU), emergency room (ER), operating room (OR), and recovery room (RR).
Pulse oximeter design is well documented. It utilizes two light-emitting diodes (LED). Each LED emits a specific wavelength of light that is transmitted through the tissues to a photodetector. These wavelengths are chosen to be around 660 nm (red spectrum) and around 940 nm (near-infrared spectrum) because of the absorbency characteristics of oxyhemoglobin (HbO2) and reduced hemoglobin (RHb). An electrical signal consisting of two components is generated by the photodetector receiving the LED emission. There is an invariant direct current (DC) component to the signal which represents ambient background light and transmission of light through invariant, that is, nonpulsatile tissues such as skin, bone, and, to a certain extent, veins. The second component of the signal is an alternating current (AC) which represents the varying transmission of light through the pulse-varying tissues, i.e., the arteries and capillaries. Both the AC and DC components are affected by altered LED light intensity. The AC signals must be corrected for inter-LED light intensity differences prior to their use for SpO2 calculation. A pulse oximeter does this by dividing each LED's AC signal by its corresponding DC signal to produce the "corrected AC signal." The ratio of the corrected AC signal at 660 nm to that at 940 nm is compared to a stored calibration curve that yields SpO2. Thus, to calculate SpO2, a pulse oximeter generates a corrected AC signal for both LED wavelengths.
At this time, determination of the other variables, such as methemoglobin (MetHb), Hct, THb, CaO2, continues to require arterial puncture or phlebotomy. Skin puncture procedures are painful to the patient, time consuming, and provide opportunities for infection. There is a great need for a rapid and accurate noninvasive means of assessing THb, CaO2, and Hct. Such an assessment means would enable the health care provider to quickly evaluate and follow a patient's circulating blood status. Questions of hemodilution during volume expansion in the field, ER, and OR would be rapidly answered. Hemoconcentration after blood transfusion, hemodialysis or bone marrow transplantation could be followed without repeated venipuncture. Furthermore, many "routine" screening phlebotomies to assess THb and Hct such as for preoperative laboratory studies in children and adults could be eliminated.